The invention relates generally to control techniques for vibratory equipment. More particularly, the invention relates to the use of a programmable electronic control system for vibratory equipment that includes pulse width modulated drive signals, vibration amplitude and resonant frequency tracking.
Various material handling systems use vibration as a method for feeding and orienting parts for use in automated assembly processes. For example, a feeder bowl is a well-known device used to orient and feed parts. The bowl is mounted on a number of inclined springs, and typically vibrates in a rotational manner. Parts are dropped or otherwise placed in the bowl interior and the rotational vibration causes the parts to "walk" up a helical shelf within the bowl. Tooling is placed along the shelf to orient and/or sort the parts as needed for the next stage of the assembly process. A number of electromagnets are used to apply a mechanical force to the system to cause the vibratory motion.
The primary mode of vibration is resonance. The bowl and mounting springs together form a mass-spring mechanical system that exhibits a natural frequency of vibration or resonant frequency. Typically, the system is mechanically "tuned" by adding or removing springs so as to force by design the resonant frequency to be near 60 Hz or 120 Hz. These frequencies are selected so that standard 60 Hz outlet power can be used to drive the electromagnets.
Most feeder bowls are driven open loop, meaning that the vibration intensity is controlled simply by using a variable transformer, for example, or some other means for manually regulating the drive voltage. Such open loop control techniques tend to be sensitive to line voltage variations, load changes, spring wear and other dynamic conditions that result in inconsistent control of the vibration envelope. For applications involving larger bowls, such open loop control is not desirable. Closed loop control systems are known but tend to be application specific. Closed loop control systems typically attempt to drive the feeder bowl at its resonant frequency and use sensors for vibration amplitude feedback. The known systems tend to use expensive and complicated linear drive circuits and control techniques, and also consume excessive power.
The objectives exist, therefore, for a closed loop control apparatus and method for controlling vibration frequency and amplitude in vibratory equipment. Such apparatus, in contrast to previously known systems, preferably will be realized with minimal circuitry to reduce design and manufacturing costs and operate with lower power consumption to reduce operating costs, while providing fast response to system disturbances that produce changes in the resonant frequency and vibration amplitude. Such apparatus preferably will have universal application across a wide variety of systems, such as, for example feeder systems of substantial size variations (e.g. feeder bowls of six inch to forty-eight inch diameters) and a wide variety of vibration frequencies.